Embodiments of the present invention relate generally to gas turbine engines, and more particularly to turbine nozzles for such engines that incorporate a low-ductility material.
A typical gas turbine engine includes a turbomachinery core having a high pressure compressor, a combustor, and a high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. The high pressure turbine includes one or more stages which extract energy from the primary gas flow. Each stage comprises a stationary turbine nozzle followed by a downstream rotor carrying turbine blades. These components operate in an extremely high temperature environment, and must be cooled by air flow to ensure adequate service life. Typically, the air used for cooling is extracted (bled) from the compressor. Bleed air usage negatively impacts specific fuel consumption (“SFC”) and should generally be minimized.
Metallic turbine structures can be replaced with materials having better high-temperature capabilities, such as ceramic matrix composites (“CMCs”). The density of CMCs is approximately one-third of that of conventional metallic superalloys used in the hot section of turbine engines, so by replacing the metallic alloy with CMC while maintaining the same part geometry, the weight of the component decreases, as well as the need for cooling air flow.
While CMC materials are useful in turbine components, it is difficult to use them for some mechanical elements such as cantilevered sections, springs, thin sections, and so forth. Therefore, a CMC component will typically need to be attached or connected to metallic components, such as baffles, spring elements, or seals.
This is complicated by the fact that CMC materials have relatively low tensile ductility or low strain to failure when compared with metals. Also, CMCs have a coefficient of thermal expansion (“CTE”) approximately one-third that of superalloys, which means that a rigid joint between the two different materials induces large strains and stresses with changes in temperature, and clamping CMC and metal components together can introduce thermal stresses or open the clamp attachment. The allowable stress limits for CMCs are also lower than metal alloys which drives a need for simple and low stress design for CMC components. Finally, because of the different material compositions of CMC and metal components, traditional joining methods such as brazing and welding are not possible.
Accordingly, there is a need for an apparatus for combining CMC and other low-ductility components with metallic components that minimizes mechanical loads and thermal stresses on the CMC components.